@gadgetgrabber
because Ice Block lets you be ahead of your opponent by two turns while you can attack his face with minions and spells.
Also the goal of the deck is not board advantage. You can see this deck as a mix of Freeze Mage and Face Hunter. You stall the game by freezing your opponent (Frost Nova) and playing Ice Block to be ahead and you go face with your minions and especially your spells

Control Priest usually doesn't run Acolyte of Pain and I don't like Flash Heal that much. I would add 1 Shadow Word Pain, 1 Shadow Word Death, 1 Velen's Chosen and Ysera.
Or are the Acolytes and the Heals so important?